textproduct: Albany

WHAT HAS CHANGED

Winter Weather Advisories for southern Herkimer County has ended now that the lake effect band has weakened and shifted south.

The timing for the snow event Monday night into Tuesday has shifted slightly later with more confidence in snow arriving from southwest to northeast right at the start of the A.M commute instead of overnight.

Temperatures for Friday have trended colder and therefore precipitation type now favors more snow at the start of the event. There remains uncertainty in how and where precipitation types change during the day Friday as warm air advects from south to north.

KEY MESSAGES

1. Moderate to high confidence in the potential for a light snowfall event on Tuesday with moderate amounts in the southern Adirondacks and southern Greens. The start of the event likely impacts the morning commute with snow changing to a wintry mix by the afternoon/evening before transitioning back to snow showers overnight. There is a 50 to 75% chance for at least 2 inches of snow regionwide.

2. After a quiet stretch Christmas Eve/Day, there is increasing confidence for another disturbance on Friday with a 50-60% chance that 24-hr precipitation amounts exceed 0.50" regionwide. Temperatures have trended colder and now favor mainly snow at least for the start of the event before uncertainty in p-type increases as warmer air tries to nose northward.

DISCUSSION

KEY MESSAGE 1...

Large scale ridging builds northeastward from the Southern Plains heading into Tuesday with a conveyor belt of fast west- northwest flow along its northern periphery. An embedded shortwave in the Upper Plains results in a sfc low with an area of enhanced warm air and moisture advection ahead of its warm front leading to an organized precipitation shield that expands from the Ohio Valley into eastern NY and western New England early Tuesday morning. With shortwave ridging developing ahead of the warm air advection Monday night, the onset of the precipitation has trended slightly later and now looks to arrive from southwest to northeast 09 - 15 UTC Tuesday or beginning during the A.M commute (especially south of I-90). Snow looks to hold off for areas north of the Capital District until the tail end of the A.M commute. Initial thermal profiles favor snow across the region with a period of steady light snow lasting for a few hours due to enhanced mid-level frontogenesis; however, overall forcing looks weak so not expecting any heavy snowfall rates. In fact, strengthening southwest flow aloft looks to lead to downsloping off the Catskills which should act to weaken snowfall intensity in the Capital District, parts of the northern Catskills and mid-Hudson Valley by early afternoon. There is even potential for some patchy freezing rain where temperatures remain near or slightly below freezing as the mid- levels dry out and no longer support ice nuclei in the clouds. Meanwhile, snow continues in the Upper Hudson Valley, southern Adirondacks, and western New England (potentially mixing with sleet as warm air surges northward) with upsloping enhancing snowfall along south facing slopes in the southern Adirondacks. Current forecast predicts a general 1 to 3 inches of snow regionwide during the day Tuesday. The highest amounts are expected in the southern Adirondacks where there is a 30% chance of exceeding 4 inches and confidence is highest in needing a winter weather advisory.

The strongest forcing for ascent and axis of mid-level moisture exits to our north by 21 - 00 UTC as a dry slot punches northward allowing precipitation regionwide to decrease in coverage and intensity. With sfc temperatures remaining in the low to mid 30s in valley areas, patchy plain rain showers lingers into the evening hours while sfc temperatures in higher terrain areas of the northern/eastern Catskills, Upper Hudson Valley, and in western New England hover in the upper 20s/low 30s supporting more patchy freezing rain as forecast soundings suggest clouds are too mild to support ice nuclei. While the dry slot lingers through about 06 UTC, the main compact and rather vigorous shortwave and associated arctic cold front sweep across the Northeast from 06-12 UTC. This should result in a second period of snow showers as westerly winds turn quite gusty and usher in a chilly Canadian air mass with a 50 to 75% chance for gusts to exceed 30mph. An additional coating to 2 inches of snow is expected overnight with the highest snowfall amounts likely across the southern Adirondacks and southern Greens where westerly winds can upslope the terrain. Luckily, most of the snow showers diminish Wednesday morning as high pressure builds across the region with gusty winds decreasing by the afternoon.

KEY MESSAGE 2...

High pressure at the surface and ridging aloft build over the region for Christmas Eve into Christmas Day, resulting in tranquil weather conditions with seasonable temperatures.

We continue to keep a close eye on our next potential for widespread precipitation Friday into Saturday as the conveyor belt of fast northwesterly flow remains positioned across the Great Lakes into the Northeast with yet another shortwave tracking within it. This time, a large scale Canadian high looks to be locked across eastern Ontario into Quebec which maintains a source of chilly air to our north. As the shortwave approaches during the day on Friday, the initial surge of warm air/moisture advection ahead of it should provide enough forcing for ascent to result in another organized precipitation shield. Given the colder air mass to our north, thermal profiles once again likely support mainly snow to start. There remains some uncertainty on the overall track of the incoming sfc low which will determine how and if precipitation type changes as a further north track would allow warmer air to surge northward supporting a transition to a wintry mix while a further south track would support mainly snow. Latest guidance has trended further south with the sfc low track suggesting a colder solution; however, the spread between the 25th and 75th percentile in sfc temperatures on Friday remains quite high ranging 15 to 20 degrees so we continue to message initial snow on Friday changing to a rain/snow mix in valley areas while higher terrain areas remain all snow. Given there is a 50 to 60% chance for regionwide 24-hr precipitation amounts to exceed 0.50" from 7am Fri to 7am Sat and that initial precipitation favors snow regionwide, we are becoming concerned for slippery travel during this period, especially in the hill towns and higher terrain areas where precipitation type favors more snow. While there is an increasing consensus for a widespread and moderate precipitation event, the uncertainty is tied to temperatures and if and where precipitation types change based on the storm track and the influence of the Canadian high to the north. The 00 UTC/22 GEFS and ENS ensembles suggest a 30 to 60% chance for at least 2 inches of snow regionwide through 1AM Saturday while the GEPS show slightly lower probabilities at 20 to 40%. We will continue to monitor trends as this event can impact post- holiday travel.

AVIATION /06Z MONDAY THROUGH FRIDAY/

Through 06z Tuesday...VFR conditions expected to prevail through the 24 hour TAF period. SCT-BKN lake effect clouds will continue move across the TAF sites prior to 12z, especially KALB/KPSF in the NW flow regime. Clouds should generally dissipate by 12z as a ridge of high pressure builds in from the west. SCT-BKN mid level clouds will be around later this morning through the afternoon, with OVC skies developing this evening ahead of a clipper system starting to approach from the upper Great Lakes. Winds will initially be northwest around 8-15 kt with gusts of 20-25 kt lowering t 5-10 kt by later this morning.

Outlook...

Tuesday Night: Moderate Operational Impact. Breezy. Chance of SHRA...SHSN. Wednesday: Low Operational Impact. Breezy. NO SIG WX. Wednesday Night: No Operational Impact. NO SIG WX. Christmas Day: No Operational Impact. NO SIG WX. Thursday Night: Moderate Operational Impact. Chance of SN. Friday: High Operational Impact. Definite SN. Friday Night: Moderate Operational Impact. Likely SN. Saturday: Moderate Operational Impact. Chance of RA...SN.

ALY WATCHES/WARNINGS/ADVISORIES

CT...None. NY...None. MA...None. VT...None.